1. Field of the Invention
This invention relates to optical and electronic devices and, in particular, optical and electronic devices employing superlattice structures.
2. Art Background
Doping superlattice structures, i.e. structures having a periodic doping profile including alternating n- and p-dopant regions, have been proposed for a variety of applications including optical switches, Light Emitting Diodes (LED), lasers, and photodetectors. In each application either (1) incident light is absorbed in the superlattice structure and the resulting photogenerated electrons and holes are separated yielding concomitant optical or electronic properties, or (2) an electric field is applied to the superlattice structure to induce the emission of light or to induce a change in optical properties.
Two generic configurations for superlattice devices have been proposed. In one configuration a periodic structure composed of crystalline materials of different compositions are layered to produce regions of different chemical potential separated by potential barriers. Although this configuration has great promise, often difficulties associated with the intimate contact between crystallographically diverse materials present problems. For example, large lattice mismatches between adjacent material regions lead to high defect density and even polycrystallinity which renders the structure unsuitable for device applications. The resulting limitations have restricted suitable materials for such devices to a few specific III-V and II-VI semiconductor materials.
A second superlattice structure that avoids difficulties associated with the contact of crystallographically different regions utilizes a symmetric .delta.-doping configuration. In this configuration, a single semiconductor material such as, for example, gallium arsenide is periodically doped with alternating concentrations of donors and acceptors. Generally, a fabrication procedure such as molecular beam epitaxy (MBE) is employed. A source or sources for the semiconductor material, e.g. a gallium and an arsenic source for gallium arsenide, together with sources for acceptor and donor dopants, e.g. beryllium and silicon respectively, are used. Equally spaced alternating regions of acceptor doped gallium arsenide and donor doped gallium arsenide are deposited. In some structures these doped regions are adjoining while in other structures denominated .delta.-doped structures these regions are generally thin, e.g. a few atomic layers thick, and separated by regions of undoped gallium arsenide. In all cases, the structures are symmetric with respect to an inversion symmetry operation. Such a .delta.-doped structure yields an electronic band diagram such as shown in FIG. 1 corresponding to the structure shown where 2 denotes the valence band and 3 denotes the conduction band of the resulting device.
Structures with .delta.-doping have advantages over structures with crystallographically different compositions since difficulties associated with, for example, crystal lattice mismatch are avoided. However, in these devices, electronic and optic operating characteristics are interrelated such that improvement in one property often induces a degradation in a second. For example, the response time of the device is determined by the distance indicated by 5 in FIG. 1. The electric field distribution in the device indicated by the angle 6 in FIG. 1 controls the optical absorption and transmission properties of the structure. Clearly, the distance 5 and the angle 6 are interdependent. As a result, if the optical properties are set, the electronic properties are predetermined while similarly if the electronic properties are chosen, then the optical properties are predetermined. Thus, superlattice devices that are relatively easy to fabricate and that allow flexible control over the resulting device properties are not available.